Valence Electrons | Atomic Structure | Energy Of An Electron

atomic Structure

Atomic Structure

Valence Electrons: According to modern world theory, the matter is electrical in nature. All the materials are composed of very tiny particles called atoms. The atoms are the building bricks of all things in the visible world. An atom composed of a central nucleus of positive charge around which tiny negatively charged particles, called electrons revolve in different paths or orbits.

(1) Nucleus.

It is the central part of an atom and *composed of protons and neutrons. A proton is a positively charged, while the neutron has the nearly same mass as the proton, but has zero charges. That’s why the nucleus of an atom is positively charged. The sum of protons and neutrons constitutes all weight of an atom and is called atomic weight. It is due to the particles in the extra nucleus (i.e. electrons) have negligible weight as compared to protons or neutrons.

∴ atomic weight = no. of protons + no. of neutrons

(2) Extra nucleus.

It is the outside part of an atom and contains only electrons. An electron is a negatively charged particle having nearly zero mass. The charge on an electron is equal but opposite to on a proton. Also, the number of electrons is equal to the number of protons in an atom under normal conditions. That’s why an atom is neutral as a whole. The number of electrons or protons in an atom is known atomic number i.e.

atomic number = no. of protons or electrons in an atom

The electrons in an atom revolve around the nucleus in different orbits or paths. The number and arrangement of electrons in an orbit is found by the following rules :

(i) The number of electrons in orbit is given by 2n, 2 where n is the number of the orbit. For example,

  • First orbit contains 2 × 12 = 2 electrons
  • Second orbit contains 2 × 22 = 8 electrons
  • Third orbit contains 2 × 32 = 18 electrons

(ii) The last orbit cannot have more than 8 electrons.
(iii) The last but one orbit cannot have more than 18 electrons

Structure of Elements

We know that all atoms are made up of protons, neutrons, and electrons. The difference between many types of elements is because of the different number and arrangement of these particles within their atoms. For example, the structure* of a copper atom is more different from that of a structure of carbon atom and hence the two elements have different properties. The atomic structure can be easily made up if we know the atomic weight as well as the atomic number of the element. Thus taking the case of the copper atom,

Structure of Elements

Atomic weight = 64
Atomic number = 29
∴ No. of protons = No. of electrons = 29
and No. of neutrons = 64 − 29 = 35

Fig. 1.4 shows the structure of the copper atom. It has 29 electrons which are placed in different orbits as follows. The first orbit always has 2 electrons, the second 8 electrons, the third 18 electrons, and the fourth orbit has 1 electron. The atomic structure of all discovered elements can be shown in this way and the reader is advised to try for a few commonly used elements.

Energy Of An Electron: Since electronics deals with small particles called electrons, these tiny particles require a more detailed study. As discussed before, an electron is a negatively charged particle having nearly zero mass. Some of the important most properties of an electron are :

  1. Charge on an electron, e = 1.602 × 10−19 coulomb
  2. Mass of an electron, m = 9.0 × 10−31 kg
  3. Radius of an electron, r = 1.9 × 10−15 metre

The ratio e/m of an electron is 1.77 × 1011 coulombs/kg. This actually means that the mass of an electron is very small as compared to its charge. It’s because of this property of an electron that it is very mobile and is huge influenced by electric or magnetic fields.

Energy of an Electron

energy of an Electron

An electron rotating around the nucleus possesses two kinds of energies viz. kinetic energy because of its motion and potential energy because of the charge on the nucleus. The net energy of the electron is the sum of these two energies. The energy of an electron increases when its distance from the nucleus increases. Thus, an electron in the second orbit has more energy than the electron in the first orbit; electron in the third orbit has larger energy than in the second orbit. It is clear that electrons in the last orbit must have very high energy as compared to the electrons in the internal orbits. These last orbit electrons play a very important role in finding the physical, chemical, and electrical properties of a material.

Valence Electrons

The electrons in the outermost orbit of an atom are called valence electrons. The outermost orbit can carry a maximum of 8 electrons. The valence electrons define the physical and chemical properties of a matter. These valence electrons determine whether or not the material is chemically active; metal or non-metal or, gas or solid. These electrons also explain the electrical properties of a material. On the basis of current conductivity, materials are normally classified into conductors, insulators, and semiconductors. As a rough rule, one can determine the electrical behavior of material from the number of valence electrons as under :

(i) Whenever the number of valence electrons of an atom is less than four (i.e. half of the maximum
eight electrons), the material is normally metal and a conductor. Examples are sodium, magnesium, and aluminum which have one, two, and three valence electrons respectively (See Fig. 1.5).

Valence Electron

(ii) Whenever the number of valences shell electrons of an atom is more than four, the material is normally a non-metal and an insulator. Examples are nitrogen, sulfur, and neon which have five, six, and eight valence electrons respectively (See Fig. 1.6).

Valence Electron

(iii) Whenever the number of valences shell electrons of an atom is four (i.e. exactly one-half of the maximum 8 electrons), the material has both properties of metal and non-metal and is usually a semiconductor. Examples are carbon, silicon, and germanium (See Fig. 1.7).

Valence Electron

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Reference : Principles Of Electronics Multicolor Illustrative Edition By V K Mehta And Rohit Mehta

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